This invention relates generally to demountable printing cylinders, and more particularly to a mandrel assembly for supporting a demountable cylinder and including a pair of hydraulically-actuated sleeves which are expansible to engage the end heads of the cylinder to lock the cylinder to the mandrel assembly, the assembly being operable in small as well as large diameter sizes.
In gravure printing, use is made of a printing cylinder whose surface is etched with cup-like cells which, as the cylinder passes through an ink fountain, pick up and carry the ink. When the cylinder engages an impression roller, the ink is transferred to the surface of the paper running therebetween. Flexographic printing uses similar inks, but the ink is picked up by rubber printing plates attached to a cylinder.
Since in the course of such printing operations, it is frequently necessary to replace one cylinder by another, various expedients have heretofore been proposed to provide demountable cylinder structures whereby the same mandrel may be coupled to different cylinders for use in the printing machine.
The simplest mechanical expedient for this purpose is set-screws to attach a cylinder to the mandrel. While set-screw arrangements are uncomplicated, they may have serious practical drawbacks. It is difficult to achieve proper concentricity with set screws; and as a consequence, the printing is of poor quality. Moreover, set-screws tend to vibrate and work loose. Other more complicated mechanical locking devices, such as split-lock clamping collars and expanding collets, have been suggested, but these are generally more expensive and equally inaccurate.
One may obtain accurate mounting for printing cylinders using a heat-shrinkage procedure to attach and detach a cylinder to or from a mandrel. This procedure involves end closures on the cylinder having a relatively high coefficient of thermal expansion with respect to the mandrel, and it requires special heating equipment. Not only is the procedure time-consuming, but should axial of side-to-side adjustment of the cylinder on the mandrel be necessary, the heating procedure must be repeated with a further loss of time.
Another known approach makes use of hydraulically-actuated collet locks for demountable cylinders. However, known devices of this type require grease guns to pump fluid into the lock each time a locking action is to be effected, the grease being bled off each time the mandrel is to be released. The use of grease in the environment of printing operations is obviously undesirable. Moreover, it is not possible with such known devices to determine, without the use of additional expedients, the amount of hydraulic pressure that is being imposed on the mandrel, and whether it is sufficient to afford adequate torque resistance. As a consequence, cylinder creep or slippage may be encountered in the course of printing, with deleterious effect.
The Hoexter U.S. Pat. No. 3,378,902 discloses a printing cylinder having a pair of hydraulically-actuated collets mounted at opposing ends thereof, the mandrel for supporting the cylinder being slidably receivable within the collets and securely locked thereto when hydraulic pressure is applied. Each collet includes a cylindrical sleeve having a thick-walled hub section and a relatively long thin-walled pressure section. A broad circumferential channel is cut in the pressure section to form a bendable pressure wall, the pressure section being surrounded by a collar of high tensile strength whose edges are welded to the pressure section to define an annular fluid chamber bounded by the collar and the pressure wall.
A pressure cartridge is fitted into a cavity in the hub action, the cartridge communicating with the fluid chamber in the pressure section and including a piston which is advanced inwardly by an adjusting screw. When the annular chamber is filled with hydraulic fluid and the piston is advanced inwardly, the resultant hydraulic pressure causes flexure of the pressure wall, thereby subjecting the mandrel to radially-directed stresses which are uniformly distributed and serve to lock the mandrel to the collet and at the same time to maintain proper concentricity.
In the Hoexter arrangement, the mandrel is a standard shaft, but the demountable cylinder is not of standard design. It is a special cylinder which includes a pair of hydraulically-actuated and collets, as described above. Hence the special cylinder is substantially more expensive to manufacture than a standard cylinder. Since each machine in the printing facility is provided with several special cylinders each operable with a common mandrel, the overall cost of this arrangement is high.
The three piece mandrel assembly disclosed in my aboveidentified copending application has distinct practical advantages for it is useable with low cost standard demountable printing cylinders rather than with a more costly printing cylinder of unconventional design. This mandrel assembly is composed of a tube having journals joined thereto, the journals extending axially from either end of the tube.
Encircling each journal at the position thereon adjacent the end of the tube is an expansible sleeve whose normal dimensions are such that the sleeve fits neatly within the corresponding end head of the printing cylinder to be mounted on the assembly. Each journal is relieved just below the sleeve thereon to define an annular hydraulic chamber. Formed in the journal in a region adjacent the sleeve is a lateral bore within which is received a piston and a piston screw. When turned in, the screw causes the piston to advance, this advance serving to apply pressure to a hydraulic fluid filling an internal duct which communicates with the hydraulic chamber. As a consequence of this hydraulic pressure, the sleeve surrounding the chamber is caused to expand to grip the associated and head on the printing cylinder and thereby locking the cylinder to the mandrel assembly.
The internal duct disclosed in my copending application is displaced from the central axis of the journal, whereas the lateral bore which houses the piston and piston screw is at right angles thereto. The duct and bore arrangement in the journal is relatively difficult to machine. Moreover, this arrangement is not feasible for small diameter mandrel assemblies, such as one having a two and a half inch maximum diameter, for then the available space is insufficient to permit adequate piston advance in the lateral bore to develop the necessary hydraulic pressure.
Another drawback of my prior mandrel assembly in which a piston operates at right angles to the off-center duct filled with hydraulic fluid, this in turn being linked by right angle branches to the annular hydraulic chamber, is that the resultant fluid path has several sharp bends. It is not always possible with this angled path to fully bleed the air from the hydraulic system, for pockets of air may become trapped in cavities created at the bends. The bleeding of air is essential, for air is compressible and interferes with the proper action of the incompressible hydraulic fluid.